bcachefs: don't attempt rw on unfreeze when shutdown

The internal freeze mechanism in bcachefs mostly reuses the generic
rw<->ro transition code. If the fs happens to shutdown during or
after freeze, a transition back to rw can fail. This is expected,
but returning an error from the unfreeze callout prevents the
filesystem from being unfrozen.

Skip the read write transition if the fs is shutdown. This allows
the fs to unfreeze at the vfs level so writes will no longer block,
but will still fail due to the emergency read-only state of the fs.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>